1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus, such as a printer, a copying machine or the like, that transfers a toner image on a sheet and fixes the toner image on the sheet by heat.
2. Description of Related Art
In an electrophotographic image forming apparatus, generally, a sheet with a toner image transferred thereon is fed to a fixing nip portion between a fixing roller and a pressure roller. At the fixing nip portion, heat is applied to the sheet such that the toner image thereon can be fixed on the sheet. It is necessary that the fixing nip portion is maintained at a predetermined temperature. As a method of predicting the temperature of the fixing nip portion, for example, Japanese Patent Laid-Open Publication No. 2002-311750 suggests that the temperature upstream of the fixing nip portion is predicted by use of two temperature sensors, or by use of one temperature sensor and on the basis of the thermophysical property of the fixing roller.
Recently, a fixing device using an endless fixing belt made by extrusion of resin, which permits costless mass production, instead of a fixing roller has been put into practical use. Such a fixing belt has fluctuation in thickness (size in the diameterical direction) for manufacturing reasons. Accordingly, when the fixing belt is heated by a constant amount of energy, the fixing belt fluctuates in temperature depending on the position along the circumferential direction. Therefore, the temperature detection result obtained by monitoring of the temperature of a portion other than the fixing nip portion by use of a temperature sensor may be different from the temperature of the fixing nip portion. The fixing nip portion is an area nipped between the fixing belt and a pressing member, and it is impossible to directly measure the temperature of the fixing nip portion by use of a temperature sensor because of its structure. Also, when the temperature of the fixing nip portion is measured from the inner surface of the belt, the thermal capacity of the belt has an effect on the measurement result. Thus, it is impossible to measure the temperature of the nip portion accurately.